bug#15090: linum-mode and visual-line-mode (both defaults) have conflicting behavior

[Eli Zaretskii]

> Date: Wed, 14 Aug 2013 22:47:37 -0500
> From: Dave Kepler <lalop.lmao@gmail.com>
> 
> My apologies, that was a pretty serious omission.  My emacs version was
> 24.3.1.  I've just tried it with trunk and -Q, and I think I found a
> reliable way to duplicate it.
> 
> 1. emacs -Q, you start with a scratch file. Delete the text in the scratch
> file.
> 
> 2. Paste the following string, not including quotations:
> 
> "abc def ghi jkl mno pqr stu vwz yzabc def ghi jkl mno pqr stu vwz yzabc
> def ghi jkl mno pqr stu vwz yzabc def ghi jkl mno pqr stu vwz yzabc def ghi
> jkl mno pqr stu vwz yzabc def ghi jkl mno pqr stu vwz yzabc def ghi jkl mno
> pqr stu vwz yzabc def ghi jkl mno pqr abc def ghi jkl mno pqr stu vwz yzabc
> def ghi jkl mno pqr stu vwz yzabc def ghi jkl mno pqr stu vwz yzabc def ghi
> jkl mno pqr stu vwz yzabc def ghi jkl mno pqr stu vwz yzabc def ghi jkl mno
> pqr stu vwz yzabc def ghi jkl mno pqr stu vwz yzabc def ghi jkl mno pqr
> 
> 
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> 
> "
> 
> The exact string should not matter, but:
> a. The top line should be many short words.  It should be long enough to
> perform step 3.
> b. It should be followed by many empty lines
> 
> 3. Scroll down so that the top of your window breaks that top line.  That
> is, part of the top line should be above your window, so that you are only
> able to see the ending of the top line.
> 
> 4. M-x linum-mode
> 
> 5. M-x visual-line-mode
> 
> 6. Move point to the end of the top line, and type anything
> 
> 
> 
> What should happen: the text in the topmost part of the top line is unmoved
> as you append text to the end of line
> 
> What does happen: the text in the topmost part of the top line will move
> horizontally as you append text.
> 
> 
> Note: when I try creating a new file rather than using scratch, this bug
> does not happen when I append to the end of the top line.  Rather, it
> happens when I scroll using mouse.  If the bug duplication does not work as
> planned, please also try the following for step 6:
> 
> a. Scrolling with mouse
> b. Appending text far below the top line
> c. Moving point downward while at EOF
> 
> while watching the top line.  Hopefully, my duplication instructions will
> work, however.

Thanks.  This is a very old bug, now fixed in trunk revision 113889.

The reason was that we were computing the window start point
incorrectly in this situation.